1. Field of the Invention
The present invention relates to an ink jet head and an ink jet printer in which part of ink is boiled to generate bubbles to thereby eject an ink droplet from an ink jet nozzle, i.e., a thermal ink jet head and a thermal ink jet printer.
2. Description of the Related Art
In the case where an ink jet head of a thermal ink jet printer is of, for example, a top shooter type for ejecting an ink droplet substantially in a perpendicular direction to a head substrate, the ink jet head has a thin film resistor formed on a semiconductor substrate such as a silicon substrate that is a head substrate, an ink jet nozzle provided substantially above this thin film resistor, and an ink flow passage which is formed by a partitioning wall layer on the semiconductor substrate and which is in communication with this ink jet nozzle. A portion of the ink within the ink flow passage is rapidly boiled to generate bubbles to thereby eject the ink droplet from the ink jet nozzle.
In such an ink jet head, since ink is ejected by being pushed out of the ink jet nozzle by the expansion force of bubbles generated by heating the ink, the size of the ink droplet to be ejected (the amount of the ink to be ejected) depends upon the diameter of the ink jet nozzle formed or the amount of the ink within the ink jet nozzle immediately before the ink droplet is ejected, i.e., the meniscus position of the ink or the like, and the size of the ink droplet to be ejected (the amount of the ink to be ejected) from the ink jet nozzle is likely to fluctuate.
For this reason, it is desired to control the size of the ink droplet for every ink jet nozzle. However, since the ejection of the ink droplets depends upon the expansion force of the bubbles generated by boiling ink, it is difficult to control the size.
On the other hand, unlike the above-described thermal ink jet head, in the case of a so-called piezoelectric type ink jet head in which an electromechanical converter element for increasing or decreasing the volume of an ink pressure chamber by application of a drive signal is used instead of the above-described thin film resistor for ejecting the ink droplet from the ink jet nozzle, the meniscus position of ink within the ink jet nozzle is vibrated in advance by means of the electromechanical converter element so that the ink droplet is ejected in response to the timing of the meniscus vibration whereby the size of the ink droplet can be adjusted. For example, JP 2001-71538 A describes that, in the piezoelectric type ink jet head, a second voltage pulse (drive signal) can be applied in a desired meniscus position in the meniscus vibration excited by a first voltage pulse (drive signal) to eject an ink droplet to thereby adjust the size of the ink droplet.
By the way, since a thermal ink jet head in which a thin film resistor or a thin film conductive electrode is formed on a substrate by using a semiconductor manufacturing technique, in particular, a top shooter type thermal ink jet head for ejecting an ink droplet substantially in a perpendicular direction to the substrate surface, a thermal ink jet head has an advantage that the ink jet nozzles may be made on a large scale and with a high density in comparison with the piezoelectric type and, consequently, it is expected that a high-definition and high-quality image that is difficult to realize in the piezoelectric type is outputted by using the thermal ink jet head. In particular, however in order to prevent fluctuation for each ink jet nozzle, it is necessary that the size of the ink droplet to be ejected from each ink jet nozzle that has become smaller in size in order to accommodate the increase in the density of the ink jet nozzles be made uniform in order to achieve a high quality image.
However, in the thermal ink jet, since there is a problem such that the drive signal for ink ejection could not be given after the elimination of the generated bubbles, it has been impossible to desirably drive the first voltage pulse (drive signal) and the second voltage pulse (drive signal) close to each other as in the piezoelectric type but it is necessary to eject the ink in accordance with the attenuated meniscus vibration. Consequently, it is difficult to eject the ink droplet having the desired size.
Accordingly, with the thermal ink jet, it is difficult to eject the ink droplet whose size has been adjusted by controlling, as desired, the meniscus position that fluctuates in accordance with the meniscus vibration in the same manner as in the piezoelectric type. The current situation is that, in the thermal ink jet, it is difficult to output an image having the same quality as that of the image that may be realized by adjusting the size of the ink droplet in the piezoelectric system.